Leptospirosis is a life-threatening disease, which can occur in a diverse range of epidemiological situations. The spirochetal agent is a unique, albeit genetically and antigenically diverse group of bacteria divided into eight pathogenic Leptospira species and >200 serovars. The disease is considered the most widespread zoonosis in the world due to the pathogen's ability to induce a carrier state in a wide range of wild and domestic animals. Leptospires are highly motile bacteria, which can penetrate abraded skin and mucous membranes, causing a systemic infection in a short period of time by crossing tissue barriers and by haematogenous dissemination.
Spirochetes are one of the only phyla of bacteria that can be recognized and identified based on their unique and distinct corkscrew or helical morphologies. Cells of Leptospira spp., which have single short periplasmic flagella (PFs) at each end without overlapping at the center of the cell, exhibit a number of different shapes when moving. During translational motility, the anterior end is a spiral-shape and the posterior end is a hook-shaped, with translating cells rotating their PFs in opposite directions. Viewed from the center of the cell towards one of the cell ends, counterclockwise rotation results in the spiral-shaped end, and clockwise rotation results in the hook-shaped. They can readily reverse directions together with shape, and if the PFs are rotating in the same direction, the cells do not translate. When isolated in vitro and observed by negative-stain electron microscopy, PFs are extensively coiled in the form of a spring, and when the cells are at rest, fixed or dead, they differ from most other spirochetes for the presence of hook-shaped ends. Berg et al. (Berg et al., 1978, pp. 285-295, Cambridge: Cambridge University Press) proposed that in Leptospira, the PFs are more rigid when compared to the cell cylinder. Furthermore, previous studies showed that mutants that form uncoiled PFs, or without PFs, still maintain their helical shape, but have straight ends, which impairs their normal motility phenotype (Bromley et al., 1979, J Bacteriol 137, 1406-1412; Picardeau et al., 2001, Mol Microbiol 40, 189-199). Those results, taken together, indicate that the direction of rotation of PFs, and its interaction with the helically shaped cell cylinder determines a different conformation of the cell's ends, allowing them to move efficiently in both viscous media and low viscosity solvents.
PFs in spirochetes are known to be rather similar in structure and function to the flagella of other externally flagellated bacteria, as each consists of a basal body-motor complex, and a flexible hook region that connects to a helical flagellar filament. However, flagellar filaments of spirochete PFs are unique and complex, composed by multiple proteins, whereas in other swimming bacteria the Flagellin protein alone composes a thinner flagellar filament. A family of proteins named FlaB is consistently claimed to form the core of PF in spirochetes, and it shows sequence similarity with flagellin. However, spirochetes species have at least one additional set of proteins designated FlaA, which surrounds the inner core to form a sheath or partial sheath of T. pallidum, and B. hyodysenteriae PFs, but not in the sheath of B. burgdorferi PFs.
Although the function of the flagellar sheath is not clear, it has been suggested that it increases the stiffness of PFs, and that this produces a greater swimming velocity. In a more recent study, random mutants with disruption of both flaA genes in L. interrogans indicated that FlaA proteins have no involvement in the formation of the PFs sheath in this species.
Although motility and hook-deficient mutants were previously described, linking genes to protein function in these bacteria has been difficult due to the lack of tools for genetic manipulation of Leptospira. 
Therefore, a need exists in the art for understanding the composition and architecture of PFs, and the role that motility plays in leptospiral virulence.